I wouldn't call this an issue, actually if you take a closer look that space is in every other post. vBulletin makes heavy use of tables for formatting, now to put it short (and if you have a basic idea of how tables work in web pages), that blank space can be attributes to the left column vertical size (where you username, title, location, etc. is) and also to the "Report this post" line at the bottom of every post you make.

Are you sure? Seems to me I would have noticed this earlier (considering I just noticed it today without any hints from anyone and I'm hungover).
I just don't remember seeing this quantity of space after the sig before the little white line.

You had a longer signature earlier, so you didn't notice that when you were making short (less than three lines) posts. Now the leftmost column (name, avatar, place) is taller and that's why it looks like that.